1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process such as a copying machine, or in particular to image transfer onto a transfer material which affects an image quality of a hard copy.
2. Description of the Prior Art
In an electrophotographic process in an image forming apparatus such as a copying machine or a printer, a photoconductor, sensitized uniformly, is exposed according to the image data of a document, and an electrostatic latent image is formed on the photoconductor. Toners are absorbed onto the latent image by a developing device to form a visible toner image on the latent image. Then, the toner image on the surface of the photoconductor is transferred and fixed onto a transfer paper. The invention relates to the transfer of the toner image.
Such an image forming apparatus is used for reproducing various types of documents. For example, a document may include a bi-level image such as a document including characters, a document including a half-tone image such as a photograph, or a document including both characters and photographs. Then, in order to guarantee image quality for various types of documents, it is proposed, mainly in a field of a digital copying machine, to decide the attributes of a document according to the features of image data of the document, and change to image processing according to the attributes.
However, even if image processing is changed according to the attributes of image data, phenomena such as missing inside, narrowing of linear image, toner scattering, discharge noises and voids are observed. A missing inside denotes a phenomenon where a white portion with no toners happens in a black portion of a character. Narrowing denotes a phenomenon where a width of a linear image becomes narrower than the original width. Toner scattering denotes a phenomenon where toners scatter around a toner image. Discharge noises denote a phenomenon where noises happen in an area resulting in no image being reproduced in the area. A void denotes a phenomenon where a transfer of an image becomes insufficient for a dot image document or a photograph document.
In order to prevent discharge noises and toner scattering on transfer, an eraser is provided between a development position and a transfer position by exposing the entire surface of the photoconductor before transfer to remove charges on the photoconductor. This eraser is called a pretransfer eraser. Previously, an intensity of light emission of the pretransfer eraser is set at a constant value, say 5-6 mW/cm.sup.2) irrespective of document type.
The appropriate intensity of the pretransfer eraser depends on the kind of document, but it also depends on other many factors such as gradation reproduction method, image density, environment (humidity or the like) or sensitivity of the photoconductor. For example, discharge noises are liable to happen at a highlight portion in an image.
Strictly speaking, an appropriate discharging of the surface of the photoconductor, that is, the intensity of light emission of the pretransfer eraser depends on the type of a document, gradation reproduction method, a document image and the like. In general, if the amount of discharging is large, toners tend to be scattered easily around a character image, while if the amount of discharging is small, discharge noises tends to occur easily. In a usual character image, toner scattering and discharge noises are noticeable. Therefore, the amount of discharging for a character image is set at a value which can prevent discharge noises and suppress toner scattering. On the other hand, in a photograph image, toner scattering is not noticeable, but discharge noises are noticeable in a highlight portion. Therefore, it is desirable that the amount of discharging is higher for a photograph image than for a character image.
When a two-component developing material is used, carriers adhere more easily to the photoconductor with an increase in difference between the potential of a non-image area of the photoconductor and the developing bias potential of the developing device. Many edges exist in a latent image formed with the bi-level exposure method. Then, even if the above-mentioned potential difference is not large, carriers are liable to adhere due to the edge effect. Then, there is a tendency that toners are not transferred easily to form an image having erroneous white portions generated by an incomplete transfer. On the other hand, in an image formed with the multi-level exposing method, such erroneous white portions are not noticeable, but toner scattering and inside vanishing become noticeable.
Sharpness mode is provided for emphasizing the density change in an image. For a map document, a line is reproduced more clearly by using the sharpness mode. When the sharpness mode is selected, phenomena such as toner scattering, narrowing or widening of a line image are liable to happen according to the difference between the potentials of image areas and non-image areas at the transfer point.
Appropriate discharging of charges on the photoconductor, that is, the intensity of light emission of the pretransfer eraser has to be changed according to environment such as humidity and temperature. In order to solve this problem, Japanese Patent laid open Publication 2-8874/1990 proposes an image forming apparatus which controls turning on/off of the pretransfer eraser according to the change in the humidity and temperature inside the apparatus. However, because the intensity of light emission of the pretransfer eraser cannot be set at an arbitrary value, it cannot be controlled precisely according to humidity and temperature. Moreover, it is also not possible to deal with a change in environment other than humidity and temperature, for example, the amount of charges on the photoconductor or sensitivity of the photoconductor.